<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
        .con {
            width: 300px;
            height: 300px;
            background-color: gold;

            /*解决margintop塌陷*/
            /*border: 1px solid gold;*/
            
            /* 解决margintop塌陷 */
            /*overflow: hidden;*/
        }

        .box {
            width: 200px;
            height: 200px;
            background-color: red;
            margin-top: 50px;
        }

        /*清除maringtop塌陷用的选择器  */
        .clearfix:before {
            content: "";
            display: table;
        }
    </style>
</head>

<body>

    <!--margintop塌陷-->
    <div class="con clearfix">

        <div class="box"></div>

    </div>


</body>

</html>